1. Field of the Invention
This invention relates to integrated circuit varactors and more particularly to integrated circuit varactors having a wide capacitance range.
2. Description of Related Art
It is desirable to implement varactors on integrated circuits ("ICs") rather than implementing the varactors with discrete components in order to reduce the form factors and the cost of devices that require use of varactors. Disadvantageously, the IC varactors typically have unacceptably low capacitance ranges (on the order of an octave at most) and are susceptible to pickup from surrounding or neighboring IC components. This is particularly true for IC varactors that are used as part of a high frequency Voltage Controlled Oscillator ("VCO"). In a VCO, for example, it is desirable to have the ability to tune the oscillator over a wide range of frequencies. In practice the capacitance range of varactors incorporated in the VCO limits the tuning range of the VCO. Traditionally, IC varactors are formed from junction diodes. These IC varactors have upper and lower capacitance values that are separated by at most a factor of two, i.e., the ratio of the highest capacitance of the varactor to the lowest capacitance is equal to two. When these IC varactors are combined with traditional IC inductors, the tuning range of a VCO incorporating the varactor and the inductor is reduced to about 10% of a nominal frequency. In addition, disadvantageously, the capacitance of traditional IC varactors varies due to IC fabrication process variations. In particular, the capacitance of the IC varactors may vary by as much as 20% from part to part. These IC process variations are difficult if not impossible to control. Consequently, a VCO using such an IC varactor may be unable to tune to required frequencies thereby making the VCO unacceptable for use in many applications.
The prior art IC varactors are typically implemented in such a manner that an electrical junction is formed between the well and the substrate of the IC. This junction generates a non-linear capacitance. When the prior art IC varactors are employed in a VCO, the non-linear capacitance at the junction modulates the capacitance of the varactor. This, in turn, modulates the output frequency generated by the VCO. For example, if the VCO is generating a 2 GHz signal, the non-linear capacitance created at the junction between the well and substrate of the varactor may frequency modulate the signal by 100 KHz, for example. The VCO is therefore very sensitive to low frequency noise signals that often exist in radio frequency ("RF") systems. Consequently, a need exists for IC varactors that have reduced parasitic capacitance values or, at the least, linear capacitance values. The need also exists for IC varactors that have wide capacitance ranges, are easily and inexpensively manufactured, and that exhibit reproducible, reliable, and consistent capacitance properties independent of the specific manufacturing process used (i.e., independent of lot to lot variations). The present invention presents such an integrated circuit varactor and method of manufacturing the same.